Bring sugar, milk, and butter to a rolling boil in a saucepan over medium heat and boil exactly 1 minute, then remove from heat.

Whisk in peanut butter until melted and smooth.

Fold in oats with a spatula until fully coated and no dry spots remain.

Scoop onto a parchment-lined tray, flatten slightly, rest 10 minutes at room temperature, then chill 30 minutes before serving.